We hypothesize that pathologic epithelial remodeling in airways disease and airway metaplasia is caused by a deregulation of the signaling cascades that normally govern airway epithelial regeneration after injury. These deregulated cascades can then be targeted to reverse the pathologic airway epithelial changes in airway diseases. In addition, appropriately modulating these cascades could be used in a regenerative medicine approach in order to promote normal epithelial repair. The Hippo pathway is essential for epithelial repair in multiple organ systems and its aberrant activation is known to result in metaplasia and cancer. However, the signaling remains largely unexplored in the lung. We propose to study the role of the Yap1 transcriptional co-activator in the lung since Yap1 is a vita and easy to manipulate downstream effector of Hippo signaling. Of note, Yap1 and Notch genetic alterations have been identified in squamous cell lung cancer. This is of interest because Notch signaling has been shown to regulate the airway stem cells that are responsible for epithelial regeneration and Notch signaling has also been shown to interact with Hippo signaling in other organ systems. In aggregate, these observations suggest that the Hippo and Notch pathways will interact during airway regeneration and in causing airway epithelial metaplasia. Our preliminary data show that genetic activation of Yap1 in airway epithelium causes an epithelial hyperplasia resembling the early sequence of changes associated with squamous metaplasia. We also show that Yap1 expression is induced upon airway injury and then subsides as airway epithelial regeneration proceeds. We therefore hypothesize that the Hippo pathway is required for airway epithelial regeneration and that a deregulation of this signaling cascade in airway basal stem cells will cause squamous metaplasia. This proposal aims to define the mechanistic role of Hippo signaling in airway epithelial repair and remodeling by (1) modulating Yap1 expression in airway basal stem cells in both murine and human model systems and (2) by examining its relationship to the Notch pathway. Specifically, in Aim 1 we will genetically and reversibly activate Yap1 in airway basal stem cells using a doxycycline-inducible activated form of Yap1 and also test the role of Yap1 activation in both mouse and human airway epithelium ex vivo. In Aim 2, we will genetically delete Yap1 from airway basal stem cells and then challenge mice with sulfur dioxide to elicit a regenerative response to determine whether Yap1 is necessary for airway epithelial injury repair. We will also determine the consequences of Yap1 loss in mouse and human airway epithelium ex vivo. In Aim 3, we will modulate Notch and Yap1 in mouse and human airway progenitor cells to determine how one signaling pathway affects the other. The state of these signaling cascades will then be examined in samples of human squamous metaplasia and dysplasia by defining the presence or absence of downstream targets of the Hippo and Notch pathways.